1. Field of the Invention
The present invention relates to a fixing device capable of locally heating a fixing member, such as a fixing belt, etc., employed in an image forming apparatus, such as a copier, a facsimile machine, a printer, etc., and an image forming apparatus incorporating such a fixing device.
2. Description of the Background Art
Conventionally, it is known that a fixing device employed in an image forming apparatus, such as a copier, a facsimile machine, a printer, etc., includes a fixing member, such as fixing belt in a belt type fixing device, a fixing roller in a heat roller type fixing device, etc., to fix an image borne on a recording medium, such as a sheet, etc., a pressing member, such as a pressing roller, etc., and a heating device that heats the fixing member as described in Japanese Patent Application Laid Open Nos. H05-6114 and 2005-181946 (JP-H05-6114-A, and JP-2005-181946-A), respectively.
Such a heating device is categorized into two types. The first type extends over the entire width of the fixing member to uniformly heat thereof, and the second type locally heats the fixing member as described in JP-H05-6114-A and JP-2005-181946-A, respectively. In the first type, a halogen heater is utilized. In the second type, a thermal head is used.
Since heat travels from the fixing member and is stored in the pressing member due to uniform heating thereof in the first type, a heat source for heating the pressing member is frequently omitted in a fixing system.
However, since the recording medium is forcibly entirely heated in the fixing system having the first heating device, a blank portion of the recording medium, that is, a portion which does not bear any images, is needlessly heated, resulting in waste of energy.
By contrast, in a fixing system having the second heating device, since only an image bearing portion of the recording medium is heated, energy is not wasted, greatly saving power in comparison therewith.
However, since the heating device heats the fixing member so that the fixing member targets an image bearing portion of the recording medium, the pressing member hardly raises its own temperature. Consequently, heat applied to the fixing member by the heating device only for the purpose of fixing is quickly stripped off by the pressing member, thereby causing fixing malfunction.
Consequently, it is inconsistent with the purpose of reducing consumption of the power, but temperature of the pressing member may be previously increased to a prescribed level or calorie is increasingly supplied to the fixing member by the heating device. However, in the second system, when temperature of the fixing member is simply increased by the heating device from a low to high level, a difference in temperature of a boundary between the recording medium and a borne image thereon increases, resulting in insufficient or excessive glossiness even being capable of resolving the fixing malfunction. Specifically, a fixing performance and the glossiness are trade off.